This invention pertains to a silicon nitride (Si.sub.3 N.sub.4) ceramic body and a process for preparing the ceramic body.
Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance and high electrical resistivity. In addition, silicon nitride ceramics are resistant to chemical attack, particularly to oxidation. For these attributes silicon nitride is useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines.
Failure of silicon nitride ceramics is generally associated with brittleness and flaws. The object therefore is to prepare a silicon nitride ceramic with high fracture toughness (K.sub.IC) and strength. Fracture strength is directly proportional to the fracture toughness and inversely proportional to the square root of the flaw size. High fracture toughness combined with small flaw size is therefore highly desirable. Monolithic silicon nitride, however, has a relatively low fracture toughness of about 5 MPa (m).sup.1/2.
U.S. Pat. No. 4,543,345 teaches that the addition of silicon carbide whiskers to ceramic materials can result in an increase in the fracture toughness. Silicon carbide whiskers have a single crystal structure and are in a size range of about 0.6 .mu.m in diameter and about 10 .mu.m to about 80 .mu.m in length. This technique, however, does not provide significant toughening in the case of silicon nitride ceramics. Moreover, the use of silicon carbide whiskers is associated with serious processing problems. The whiskers have a tendency to agglomerate and settle. It is difficult to deagglomerate the whiskers without significantly destroying the whiskers' length. In addition, the whiskers are difficult to manufacture; thus, they display inconsistent properties and are costly. It would be highly desirable to have a silicon nitride ceramic of high fracture toughness which does not require the presence of silicon carbide whiskers.
It is known that the high temperature strength of hot-pressed silicon nitride ceramics can be increased by crystallization of the grain-boundary glass phase (second phase). This has been demonstrated in a hot-pressed composite containing beta(.beta.)-silicon nitride and a crystalline second phase of Si.sub.3 N.sub.4.Y.sub.2 O.sub.3, as reported by Akihiko Tsuge et al. in the Journal of the American Ceramics Society, 58, 323-326 (1975). However, the fracture toughness of this silicon nitride is only 5-6 MPa (m).sup.1/2.
It is also known that the presence of .beta.-silicon nitride with a high aspect ratio can increase the fracture toughness of silicon nitride ceramics, as reported by F. F. Lange, in the Journal of the American Ceramics Society, 62 (12), 1369-1374, (1983). "Aspect ratio" is defined as the ratio of the length of the whisker to the diameter or width of the whisker. Thus, whiskers with a high aspect ratio are fibrous in nature. If such whiskers are also strong, crack propagation must take a tortuous path around the whiskers, thereby leading to high fracture toughness. The transformation of alpha(.alpha.)-silicon nitride to .beta.-silicon nitride takes place above 1600.degree. C.; however, crystals of the beta phase precipitate usually as a mixture of equiaxed grains and elongated grains with a low aspect ratio. Reproducible control of the aspect ratio is a difficult problem.
Typically, the prior art is silent with regard to aspect ratio and fracture toughness of silicon nitride ceramics. U.S. Pat. No. 4,279,657, for example, discloses a powder dispersion containing silicon nitride and magnesium oxide which is hot-pressed to form a light-transmitting silicon nitride ceramic. The ceramic is disclosed to comprise more than 50 weight percent .beta.-silicon nitride ranging in grain size from 1 .mu.m to about 10 .mu.m, but typically less than 5 .mu.m. This patent teaches that impurities, such as calcium, in a total amount greater than about 0.1 weight percent are undesirable. U.S. Pat. No. 4,227,842 discloses a cutting tool consisting essentially of beta phase silicon nitride and yttrium oxide. The tool is prepared by hot-pressing a powder mixture of .alpha.-silicon nitride and yttrium oxide to obtain a ceramic of 100 percent theoretical density. U.S. Pat. No. 4,652,276 teaches a cutting tool comprising a granular phase consisting essentially of .beta.-silicon nitride and an intergranular amorphous phase consisting essentially of magnesium oxide from about 0.5 to about 10 weight percent, yttrium oxide from about 2.5 to about 10 weight percent, silicon oxide in an amount less than about 2.5 weight percent, and the balance less than 5 weight percent impurities such as aluminum. The tool is prepared by hot-pressing.
It would be very desirable to have a silicon nitride ceramic of high fracture toughness and high fracture strength. It would be advantageous if such a strong silicon nitride ceramic could be prepared without silicon carbide reinforcing whiskers. Moreover, it would be highly desirable to have a process which would be reproducible, inexpensive, and amenable to industrial scale-up for preparing such a tough and strong silicon nitride ceramic.